1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), particularly to an LCD having thin film transistors for driving each pixel.
2. Description of the Related Art
Flat panel displays such as LCDs, organic electroluminescence (EL), or plasma displays have been enthusiastically developed and commercialized in recent years. Particularly, LCDs have become the most popular display for office automation (OA) devices and audio visual (AV) devices, because LCDs have attractive features such as thin and low power consumption. Especially, active matrix LCDs employing thin film transistors (TFTs) as switching elements for controlling a timing to rewrite pixel data into each pixel enable a wide screen and animation display with a high resolution, and they are now widely used in television sets, personal computers, mobile computers, and monitors of digital still and video cameras.
A TFT is a kind of field effect transistor (FET) made of metal and semiconductor layers formed in a predetermined pattern on an insulated substrate. In an active matrix LCD, each TFT is connected to a corresponding capacitor for driving the liquid crystal disposed between a pair of substrates; the capacitor is constructed between the substrates.
FIG. 1 is an enlarged plan view of a display pixel portion of an LCD, and FIG. 2 is a cross section of the LCD along Bxe2x80x94B line shown in FIG. 1. On the substrate 50 a gate electrode 51 is formed that is made of Cr, Ti, Ta, or another suitable metal, over which a gate insulating film 52 is formed. On the gate insulating film 52 an amorphous silicon, i.e., a-Si film 53 is formed in an island shape so as to cross over the gate electrode 51. On the a-Si film 53 an N+ a-Si film 53N is formed, each end of which is doped with impurities so as to make an ohmic layer. Above the channel region of the a-Si film 53, an etch stopper 54 is remained. On the N+ a-Si film 53N a drain electrode 56 and a source electrode 57 are formed, over which an interlayer insulating film 58 is formed. On the interlayer insulating film 58 a pixel electrode 59 made of indium tin oxide (ITO) or Al is formed, and is connected to the source electrode 57 via a contact hole formed in the interlayer insulating film 58. On the pixel electrode 59, an alignment film 71 made of polyimide or the like is formed, and is processed by rubbing treatment as shown in FIG. 3. In this way, the TFT substrate is manufactured.
On another substrate 60 facing the TFT substrate 50, red (R), green (G), and blue (B) color filters 61 are formed, each of which is made of a film resist and is disposed at a position corresponding to each pixel electrode 59. In addition, a black matrix 61BM which is made of a light shielding film resist is formed between the color filters 61 without clearance at a position corresponding to a gap between the pixel electrodes 59 and at a position corresponding to the TFT. On the layers of these color filters 61 a common electrode 62 made of ITO is formed. On the common electrode 62 an alignment film 72 is formed in the same way as on the substrate 50 side and is processed by rubbing treatment as shown in FIG. 4. In this way, the opposing substrate is manufactured.
Between the TFT substrate 50 and the opposing substrate 60, a liquid crystal layer 80 is disposed. The orientation, i.e., the alignment of the liquid crystal molecules 81, is controlled in accordance with an intensity of an electric field formed by a voltage applied between the pixel electrodes 59 and the common electrode 62. Outsides of the substrates 50 and 60 polarizing films (not shown) with perpendicular polarizing axes are provided. Linear polarized light passing through these polarizing films is modulated when passing through the liquid crystal layer 80 that is controlled in different alignment per each display pixel, and is thereby controlled in a desired transmittance.
In the above-mentioned example, the liquid crystal has a negative dielectric constant anisotropy. The alignment films 71 and 72 are vertical alignment films that control the initial alignment of the liquid crystal in the direction perpendicular to the substrate. In this case, when a voltage is not applied, the linear polarized light that passed through one of the polarizing films is blocked by the other polarizing films after passing through the liquid crystal layer 80 so that the display is recognized as black. When the voltage is applied, the linear polarized light that passed through one of the polarizing films is double refracted by the liquid crystal layer 80 to become an elliptically polarized light, which passes the other polarizing films so that the display is recognized as nearly white. This type is called a normally black (NB) mode. Particularly, the vertical alignment films 71 and 72 are processed by the rubbing treatment, so that the initial orientations of the liquid crystal molecules 81 are aligned in the direction with a slight pretilt from the normal direction. This pretilt angle xcex8 is normally set to more than one degree, but equal to or less than five degrees. The liquid crystal molecule 81 is electrically uniaxial. The angle between the axial direction and the direction of the electric field is determined by the electric field strength, while the azimuth with respect to the direction of the electric field is not controlled. The liquid crystal molecule 81 having the negative dielectric constant anisotropy tilts in a direction different from the electric field direction. However, by providing pretilt, an applied voltage can make the liquid crystal molecule 81 tilt toward the pretilt direction. Thus, giving the pretilt angle and controlling the tilt direction of the liquid crystal molecule 81 to be aligned, a variation of alignments of the liquid crystal in a plane can be suppressed and deterioration of the display quality can be prevented.
The black matrix 61BM is provided for preventing light passing due to a birefringence of the liquid crystal with the pretilt in a region in which the voltage is not applied between the display pixels.
The liquid crystal having a negative dielectric constant anisotropy changes the alignment of its molecules upon the electric field, in such a way that the alignment becomes perpendicular to the direction of the electric field. On this occasion, the liquid crystal generates an action opposing the generated electric field. However, in general, such a change of the alignment from the vertical alignment of the liquid crystal is not stable compared with the case a liquid crystal having a positive dielectric constant anisotropy such as a twist nematic (TN) liquid crystal changes from the horizontal alignment. Especially, unevenness of the alignment film 71 and 72 at the interface with the liquid crystal due to a step of the TFT or the color filter layer influences the alignment change of the liquid crystal, resulting in a deteriorated display quality.
Furthermore, as shown in FIGS. 3 and 4, the related art uses rubbing treatment of the vertical alignment film 71 and 72 in order to give the pretilt xcex8 to the initial alignment of the liquid crystal as shown in FIG. 2. Therefore, when a voltage is applied, all the liquid crystal molecules 81 tilt in the direction of the pretilt (rightward in FIG. 2). Accordingly, the tilt angle of the liquid crystal molecule 81 with respect to the optical path when viewing the LCD from upper right in FIG. 2 is different from that when viewing the LCD from upper left, resulting in different transmittances. Thus, there is a problem that a brightness or a contrast ratio changes in accordance with a viewing direction. This is known as viewing angle dependence.
Furthermore, since the black matrix 61BM formed on the opposing substrate 60 side should completely cover the gap region between the pixel electrodes, it is formed larger to allow for possible position shift when the black matrix 61BM is affixed to the TFT substrate 50 side. For this reason, effective display area decreases, and aperture ratio decreases.
In addition, the rubbing treatment for making the vertical alignment film 71 of the TFT substrate side may cause an electrostatic breakdown of the TFT, which results in defective display or decline of yield in production of LCDs.
An object of the present invention is to solve the above-mentioned problems. The liquid crystal display according to the present invention has first and second substrates facing each other, the outer surface of the first and/or second substrate being provided with a polarizing film, and the liquid crystal disposed between the first and second substrates for modulating light that passed through the polarizing film so as to perform display. The liquid crystal display further includes a plurality of thin film transistors disposed in a matrix on the surface of the first substrate facing the second substrate and electrode wires thereof, an insulating film having a flattened surface and covering the thin film transistors and the electrode wires thereof, a pixel electrode for driving the liquid crystal that is formed on the insulating film and is connected to the thin film transistor via a opening formed in the insulating film, a vertical alignment film formed on the pixel electrodes, a common electrode for driving the liquid crystal formed on the surface of the second substrate facing the first substrate, a direction control window that is electrode-free portion formed in the area of the common electrode facing the pixel electrode, a light shielding film facing only the thin film transistors, and a vertical alignment film formed on the common electrode. The liquid crystal has negative dielectric constant anisotropy, and initial alignment of the liquid crystal is within one degree from the normal direction of the substrates.
In another aspect of the present invention, a liquid crystal display has first and second substrates facing each other, the outer surface of the first and/or second substrate being provided with a polarizing film, and the liquid crystal disposed between the first and second substrates for modulating light that passed through the polarizing film so as to perform display. The liquid crystal display further includes a plurality of thin film transistors disposed in a matrix on the surface of the first substrate facing the second substrate and electrode wires thereof, an insulating film having a flattened surface and covering the thin film transistors and the electrode wires thereof, a pixel electrode for driving the liquid crystal that is formed on the insulating film and is connected to the thin film transistor via a opening formed in the insulating film, a vertical alignment film formed on the pixel electrodes, a common electrode for driving the liquid crystal formed on the surface of the second substrate facing the first substrate, a direction control window that is electrode-free portion formed in the area of the common electrode facing the pixel electrode, a light shielding film facing a gap region between the pixel electrodes and is smaller than the gap between the pixel electrodes, and a vertical alignment film formed on the common electrode. The liquid crystal has negative dielectric constant anisotropy, and initial alignment of the liquid crystal is within one degree from the normal direction of the substrates.
As described above, since the region where the thin film transistor is formed is shielded from light, a leakage current is prevented from flowing through the transistor. In addition, the other region does not need to form a light shielding film. Even if the light shielding film is formed, it is not required to be a large film considering a possible position shift in affixing two substrates to each other. Thus, an aperture ratio of each pixel can be improved.
Furthermore, in the present invention, tilt in the alignment of the liquid crystal can be controlled in the region of the slanting electric field generated at the edge portion of the picture electrode and in the no electric field region, so that the pixel division is performed properly and the viewing angle dependence is reduced.
In another aspect of the present invention, the vertical alignment film is not processed by rubbing treatment. Therefore, the initial alignment of the liquid crystal is controlled within one degree from the normal direction of the substrates, and the alignment control of the liquid crystal is performed properly without any disturbance at the edge portion of the pixel electrode and in the direction control window.
In another aspect of the present invention, the second substrate is transparent in at least the region corresponding to the pixel electrode and the region corresponding to the gap between the pixel electrodes, and at least a part of the region corresponding to the gap between the pixel electrodes is shielded from light by the liquid crystal and the polarizing film.
Thus, the light shielding film is not required to be larger than the gap between the pixel electrodes considering the possible position shift in affixing the first and second substrates to each other, so that the effective display area is increased and the aperture ration also increases.
In another aspect of the present invention, the insulating film has a thickness equal to or more than one micrometer.
Thus, the alignment control action of the liquid crystal at the edge portion of the pixel electrode and in the direction control window is not disturbed by the influence of the electric field in the thin film transistor and its electrode wire, so that pixel division can be properly performed.
As explained above, in the present invention, the light shielding film can be eliminated or at least can be minimized, so that the aperture ratio of each pixel can be improved and a bright display can be attained. In addition, the pixel division is performed properly by the electric field control so that the viewing angle dependence is reduced and the display quality is improved. Furthermore, since the rubbing process is eliminated, the production cost is reduced and electrostatic generation is prevented, so that the yield in production is improved.